Roles of cortex and mitotic apparatus in cytokinesis were analyzed in cleavage of sea urchin eggs and in polar-body formation of starfish oocytes. 1. By measuring distances among microvilli seen on the cell surface in sea urchin eggs from metaphase through telophase, it was found that contraction of the cell surface at the equatorial region of the cell starts simultaneously with the start of elongation of the cell in advance of the formation of cleavage furrow. 2. The cell shape was scarcely changed by moving mitotic apparatus with a microneedle inserted into the cell or by applying a centrifugal force to the cell, although the mitotic apparatus was significantly deformed by contact with cortex. This fact indicates the cortex is sufficiently rigid as compared with the mitotic apparatus, and therefore, that the cleavage process is scarcely affected by active or passive deformation of the mitotic apparatus during cleavage. 3. Protoplasm of the mitotic apparatus in sea urchin eggs was transplanted to the subcortical regions of the same or different cells to demonstrate factor(s) responsible for cleavage stimulus in mitotic apparatus. Neither contraction nor expansion of the cortex was found. 4. A trial was made to isolate ghost cells or a fragment of the cortex with furrow-forming activity from dividing sea urchin eggs, by manipulating the cells in an artificial medium. The trial was partially succeeded, though chemical composition of the artificial medium in which the cleavage furrow was always formed in the ghost cells and isolated cortex could not be determined. 5. Intracellular concentration of calcium ions was measured in sea urchin eggs before and during cleavage and in starfish oocytes during maturation by measuring luminescence of aequorin microinjected into the cell. No distinct change in the intracellular calcium ion concentration accompanying mitotic activity of the cell was found.